1. Field of the Invention
The present invention concerns a method to generate an angiographic magnetic resonance image of an examination region and a magnetic resonance system for implementing such a method.
2. Description of the Prior Art
In magnetic resonance tomography there are multiple possibilities to show the blood vessels in angiograms that can either be based on a contrast agent-intensified signal acquisition, or acquired without contrast agent by using the effect of flowing magnetization during the image acquisition. Since not all examined persons tolerate the administration of contrast agent, magnetic resonance angiography that is not contrast agent-intensified is gaining importance.
In magnetic resonance angiography it is typically desirable to show only the arteries and to suppress the MR signal from the veins. In MR angiography that is not contrast agent-intensified, the method that is used depends on the desired examination region, i.e. on the blood flow conditions in this region. In peripheral body regions (for example the lower legs), the blood flow is typically slow. For MR angiography the pulsation of the arterial blood flow is used in order to generate MR angiograms that are not contrast agent-intensified. Typically an image acquisition technique is used in which blood delivers a high signal, i.e. a T2-weighted or T2/T1-weighted imaging frequency that is sensitive to flow. In such flow-sensitive imaging sequences, a quickly flowing magnetization delivers little signal; the vessels are dark. Such MR images with dark vessels are acquired in a data set in which the arterial flow is high, for example in the systoles of the cardiac cycle, which leads to an MR image with dark arteries, as desired.
Furthermore, an additional MR data set is acquired, for example in the diastoles during which the arteries ideally exhibit no flow or only a very slight flow, which leads to an MR image with bright arteries. By subtraction of the MR images that are acquired from the first and second data set, an MR angiography image is obtained that shows only the arteries. In the prior art it is known to use fast spin echo sequences for this purpose, for example, as is described in (among others) Miyazaki et al. in “Non-Contrast-Enhanced MR Angiography Using 3D ECG-Synchronized Half-Fourier Fast Spin Echo”, Journal of Magnetic Resonance Imaging 12(5): 776-783, 2000. This angiography technique could be improved by, for example, flow spoiler gradients being added in the readout direction, so the flowing spins are additionally dephased, which additionally intensifies the signal obliteration in the arteries in the acquisition during the diastole (see Miyazaki et al. “Peripheral MR Angiography: Separation of Arteries from Veins with Flow-spoiled Gradient Pulses in Electrocardiography-triggered Three-dimensional Half-Fourier Fast Spin-Echo Imaging”, Radiology 227(3): 890-896, 2003). It is likewise known to use gradient echo-based imaging sequences for such angiography methods, for example sequences known as TrueFISP sequences in which the transverse magnetization is refocused by gradient moments in all spatial directions. In such TrueFISP-based methods, the necessary flow sensitization is achieved by a dephasing preparation before every data acquisition in the systole, as is described in (among others) Koktzoglou et al. in “Diffusion-Prepared Segmented Steady-State Free Precession: Application to 3D Black-Blood Cardiovascular Magnetic Resonance of the Thoracic Aorta and Carotid Arterio Walls”, Journal of Cardiovascular Magnetic Resonance 9(1): 33-42, 2007, and in Priest et al. in Proceedings ISMRM Toronto, Number 727, 2008. These methods, however, have the following disadvantage.
First, it has been shown in practice that it is very difficult for many patients to find a cardiac phase in which absolutely no flow occurs, such that the MR image with the bright flow signal often has regions with less signal or without signal in the arteries. This particularly applies for patients with rapid heart rates. Second, this type of angiography imaging is problematical, in particular in regions with fast flow. For example, with the aforementioned technique good results (i.e. good MR angiography images) can be achieved in the lower feet while it is more difficult to achieve the same result quality in the upper feet or the pelvis. The problem of signal obliterations in the arteries with high signal also occurs for patients with irregular heart beat since it is difficult to precisely determine the phase of the rapid flow during the systole and the phase of the low flow during the diastole.
In “Highly Accelerated Contrast-Enhanced MR Angiography using Ghost imaging” by R. R. Edelman et al. in Proc. Intl. Soc. Mag. Reson. Med. 17, S. 272, 2009, it is described that a mixed raw data set is generated instead of a subtraction of the signals in the two cardiac phases. In this method (in the simplest 3D execution variant) the even k-space lines (for example in the kz-direction, thus in the slice direction) are filled with data set 1 and the odd k-space lines are filled with data set 2; the transformation can thereby already be implemented or not in the readout direction and phase coding direction. A modulation of signal portions in which the two data sets differ is hereby generated in this direction. After transformation into image space in the kz-direction or z-direction (typically by Fourier transformation), a doubled 3D data set is obtained, so to speak, in which both a type of averaged original image and a second image exist, the second image being spatially separate (i.e. displaced in the z-direction as what is known as a “ghost image”) and representing the differences of the two data sets. This new method for combination of two data sets is described as advantageous compared to a traditional subtraction, particularly given the use of high acceleration factors in parallel imaging.
A third disadvantage is that the strong pulsing of the vessels leads to changes in the vessel diameter, which can lead to false results in taking the difference of the images of the systole and the diastole. A fourth disadvantage of the method described above is that EKG triggering is necessary during the image acquisition in order to correlate the image acquisition with the heart beat for the acquisition of the MR images during the systole and the diastole.
In fast imaging sequences based on spin echo, the inherent flow sensitivity is greatest in the direction of the readout gradient. One possibility to reduce the large flow effects in the image acquisition is to select the phase coding gradients along the flow direction. However, in this case it is more difficult to achieve a sufficient flow sensitivity in the data set in which the vessels should be shown dark. Moreover, in most applications the phase coding direction runs in the head-foot direction. In such acquisitions the body of the examined person runs further outside of the imaged field of view (FOV) so that the problem can occur that signals from outside of the field of view are detected, or what is known as phase oversampling must be used in order to prevent aliasing artifacts.